The present invention relates to a method of and an apparatus for the continuous casting of steel. More particularly this invention concerns a system for electromagnetically mixing the molten steel in a continuous-casting process.
In the continuous casting of metal an upwardly and downwardly open tubular mold is used into which the molten metal is poured. The walls or sides of the mold are cooled so that the metal solidifies in this mold at least in an outer skin before it withdraws from the lower end of the mold. The interior of the workpiece in the mold at least is still liquid and forms a so-called crater. The sides of the mold are normally made of highly conductive material such as copper in order to maximize heat exchange between the casting being continuously formed and the mold which itself is normally cooled by means of water.
The main problem in continuous-casting operations is that due to thermal contraction the skin of the casting separates from the mold so that the rate of heat extraction from the casting is relatively low. Furthermore, the casting produced by this method often is inadequately homogeneous so that it cannot be used for the production of many types of metals, in particular steel.
It has been found possible to partly solve some of these problems by providing on at least one of the sides of the mold a heavy-duty inductor whose magnetic field passes through the casting and produces eddy currents that serve to mix the molten metal in the casting, thereby increasing homogenity of the casting so produced and also augmenting heat exchange. Thus it is possible to produce a high-quality bloom, slab, or billet even with steels that have hitherto often been considered unsuitable for continuous casting.
In most arrangements the molten steel is introduced into the continuous-casting mold by means of a conduit whose lower outlet end lies below the surface of the metal in the mold. In order to maximize mixing or circulation within the mold such a conduit is normally provided with laterally opening orifices so that the molten metal is squirted laterally out of the conduit into the body in the mold. This augments the standard convection currents in the mold so that the impurities in the circulating metal are brought to the surface and there trapped in the layer of slag on the melt.
In practice such a system is not highly effective. Almost invariably dead regions are left in the mold where there is little circulation so that any impurities carried by the steel at this point will be left in the casting as inclusions that considerably impair the quality of the metal. Since the molten metal is inherently introduced into the mold at a relatively slow rate it is impossible to obtain by simple convection or flow currents caused by the injection direction sufficient circulation within the mold to cause most of the impurities to be trapped in the slag.
The above-described system using inductors which mix the melt does somewhat increase the exchange of impurities from the metal to the slag. Nontheless, it is noted that very frequently the melt at side regions of the mold in systems wherein the metal-introducing conduit opens toward the end walls is insufficiently mixed so that the resultant casting has many quality-impairing inclusions.